Since the implementation of universal vaccination in 1984, the chronic HBV carier rate in our general population reduced from 15-20%, down to < 1% in the post-vaccination population. However, children born to HBeAg positive mothers still may be infected with HBV despite immunization. To further reducing the HBV infection in our people, strategies in reducing infection rate in this high risk group are mandatory. Previous small scale studies using lamivudine treatment in pregnant woman in the third trimester has proved effective in reducing children infection rate. The aims of the present study are to conduct a clinical trial in using Tenofovir (category B) to reduce mother-to-infant transmission, and to monitor the hepaitits B viral status and mother hepatitis occurrence. The clinical trials will screen cases of HBsAg positive pregnant women aged 20 to 40 years at gestational at 20-32 weeks. They will be tested for HBsAg and HBeAg. In whom both markers are positive, HBV viral load will be tested. An estimated 180 pregnant women with high HBV viral load (>10^8 copies/mL) will be recruited in the study; including 80-100 subjects treated with Tenofovir 300 mg daily starting from 30-32 weeks of gestation (3rd trimester) and continued to 1 month after delivery; and 80-100 pregnant women are enrolled as controls with no drug given to the mother. The newborn babies are given with HBIG within 24 hours after delivery, and HBV vaccines at 0, 1 and 6 months. Maternal complete blood count (CBC) data tested in the first prenatal examination will be recorded. Plasma AST、ALT levels and HBV DNA are tested before Tenofovir treatment, 1 month after treatment, at the time of delivery, and at 1, 2, 4 and 6 months after delivery. HBsAg、HBeAg、anti-HBs and AST、ALT are tested in the children at day 1, 6 moths and 1 year after birth. The primary outcome is reduction of the HBsAg carrier rate of the children at 6 months of age. The secondary outcome is HBsAg carrier rate of the children at 12 months of age, the change of liver function, HBeAg, and viral load in pregnant mother after treatment.